The present invention relates to vehicle detectors which detect the passage or presence of a vehicle over a defined area of a roadway. In particular, the present invention relates to improved methods of environmental adaption of vehicle detectors.
Inductive sensors are used for a wide variety of detection systems. For example, inductive sensors are used in systems which detect the presence of conductive or ferromagnetic articles within a specified area. Vehicle detectors are a common type of detection system in which inductive sensors are used.
Vehicle detectors are used in traffic control systems to provide input data required by a controller to control signal lights. Vehicle detectors are connected to one or more inductive sensors and operate on the principle of an inductance change caused by the movement of a vehicle in the vicinity of an inductive sensor. The inductive sensor can take a number of different forms, but commonly is a wire loop which is buried in the roadway and which acts as an inductor.
The vehicle detector generally includes circuitry which operates in conjunction with the inductive sensor to measure changes in inductance and to provide output signals as a function of those inductance changes. The vehicle detector includes an oscillator circuit which produces an oscillator output signal having a frequency which is dependent on sensor inductance. The sensor inductance is in turn dependent on whether the inductive sensor is loaded by the presence of a vehicle. The sensor is driven as a part of a resonant circuit of the oscillator. The vehicle detector measures changes in inductance in the sensor by monitoring the frequency of the oscillator output signal.
Examples of vehicle detectors are shown, for example, in U.S. Pat. No. 3,943,339 (Koerner et al.) and in U.S. Pat. No. 3,989,932 (Koerner).
Detection of a vehicle is accomplished by comparing a measured value based on the oscillator frequency to a reference value. The reference value should be equivalent to the measured value when the sensor area is unoccupied. If the vehicle detector has an incorrect reference value, errors in detection may occur. These errors may result in vehicles over the sensor not being detected, vehicles being detected when the sensor area is actually empty, and a single vehicle being detected as multiple vehicles.
Vehicle detectors in use today use relatively blind approaches to adjusting the reference value in an attempt to track oscillator frequency changes caused by the environment rather than by vehicles. The methods of adjusting the reference value utilized in prior art detectors include: adjusting the reference value toward the current measurement value by a fixed amount during each fixed time interval; adjusting the reference value toward the current frequency measurement value by a fraction of the difference between the two during each fixed time interval; adjusting the reference value immediately to the current measurement value if the current frequency decreases for a predetermined amount of time; utilizing an alternative amount of adjustment of the reference value per fixed time interval when a vehicle is over the sensor; and setting the reference value to the current measurement value a fixed amount of time after the vehicle is no longer detected. Prior art vehicle detectors use various combinations of these approaches. An example of environmental tracking in vehicle detectors is U.S. Pat. No. 4,862,162 (Duley). Each of these approaches results in a high probability that the reference value will be set to the wrong value, particularly during heavy traffic when it is most important that it be set correctly.